Integrated circuits typically include a leadframe and an integrated circuit (IC) die with encapsulating material surrounding the leadframe and the IC die. The leadframe includes a die pad and a plurality of pins. Soft solder may be used to attach the IC die to the die pad. Soft solder is used for thermal and electrical conductivity performance, particularly for automotive devices.
A high process temperature is needed to melt the soft solder for IC die attachment. Soft solder has a tendency to overflow beyond the area of the IC die being attached to the leadframe. This is due to the wetting between the copper leadframe and the soft solder. Solder overflow may short the pins. For a multi-chip integrated circuit, soft solder for a second IC die may overflow to the soft solder for a first IC die with re-melting when attaching the second IC die.
To prevent solder overflow, the leadframe may be formed with side walls and v-grooves. A drawback of using a leadframe package with side walls and v-grooves is that the IC die may not be level when attached to the die pad. This causes the IC die to be tilted when a thickness of the soft solder (i.e., bond line) is not uniform across the die pad. Another drawback is movement of the IC die after placement on the soft solder due to solder flow back effect.
One approach for restricting movement of the soft solder during the heating process is disclosed in U.S. published patent application no. 2002/0056894. A die pad includes a plurality of slots that extend through the die pad to restrict the soft solder. A restrictive region is defined by the slots such that the soft solder is restricted within the restrictive region. The IC die is positioned on the restrictive region. The solder paste does not flow into the slots because of the cohesion of the soft solder. As a result, the soft solder may not flow and expand during the heating process.
Another approach for preventing solder overflow from shorting the pins is disclosed in an article titled “Solder Short Reject Reduction in Soft Solder Die Attach (SSD) Process using Mechanical Solder Shield Design,” by Rashid et al., Applied Mechanics and Material, Vol. 301 (2013), pp 127-131. A mechanical solder shield is integrated to the soft solder dispenser die attach machine to prevent the molten solder overflows to the lead area of the device.